Acta Myologica • 2014; XXXIII: p. 1-12
Genetic basis of limb-girdle muscular dystrophies: the 2014 update Vincenzo Nigro and Marco Savarese Dipartimento di Biochimica, Biofisica e Patologia Generale, Seconda Università degli Studi di Napoli and Telethon Institute of Genetics and Medicine (TIGEM), Naples, Italy
and the respiratory muscles. The clinical course and the expressivity may be variable, ranging from severe forms with rapid onset and progression to very mild forms allowing affected people to have fairly normal life spans and activity levels (1). The term LGMD is becoming descriptive and also comprises clinical pictures of different diseases. The original definition was given as muscular dystrophies milder that DMD and inherited as autosomal traits (2). However, the most severe forms with childhood onset also result in dramatic physical weakness and a shortened life-span. The advent of next generation sequencing approaches has accelerated the pace of discovery of new LGMD genes. Ten years ago the list included 16 loci (3), while today the LGMD loci so far identified are thirty-one, eight autosomal dominant and 23 autosomal recessive.
Limb-girdle muscular dystrophies (LGMD) are a highly heterogeneous group of muscle disorders, which first affect the voluntary muscles of the hip and shoulder areas. The definition is highly descriptive and less ambiguous by exclusion: non-Xlinked, non-FSH, non-myotonic, non-distal, nonsyndromic, and non-congenital. At present, the genetic classification is becoming too complex, since the acronym LGMD has also been used for a number of other myopathic disorders with overlapping phenotypes. Today, the list of genes to be screened is too large for the gene-by-gene approach and it is well suited for targeted next generation sequencing (NGS) panels that should include any gene that has been so far associated with a clinical picture of LGMD. The present review has the aim of recapitulating the genetic basis of LGMD ordering and of proposing a nomenclature for the orphan forms. This is useful given the pace of new discoveries. Thity-one loci have been identified so far, eight autosomal dominant and 23 autosomal recessive. The dominant forms (LGMD1) are: LGMD1A (myotilin), LGMD1B (lamin A/C), LGMD1C (caveolin 3), LGMD1D (DNAJB6), LGMD1E (desmin), LGMD1F (transportin 3), LGMD1G (HNRPDL), LGMD1H (chr. 3). The autosomal recessive forms (LGMD2) are: LGMD2A (calpain 3), LGMD2B (dysferlin), LGMD2C (γ sarcoglycan), LGMD2D (α sarcoglycan), LGMD2E (β sarcoglycan), LGMD2F (δ sarcoglycan), LGMD2G (telethonin), LGMD2H (TRIM32), LGMD2I (FKRP), LGMD2J (titin), LGMD2K (POMT1), LGMD2L (anoctamin 5), LGMD2M (fukutin), LGMD2N (POMT2), LGMD2O (POMTnG1), LGMD2P (dystroglycan), LGMD2Q (plectin), LGMD2R (desmin), LGMD2S (TRAPPC11), LGMD2T (GMPPB), LGMD2U (ISPD), LGMD2V (Glucosidase, alpha ), LGMD2W (PINCH2).
Autosomal dominant LGMD The LGMD1, i.e. the autosomal dominant forms, have usually an adult-onset and are milder, because affected parents are usually in quite good health at reproductive age. They are relatively rare representing less than 10% of all LGMD. Sometimes, they correspond to particular cases of mutations in genes involved in other disorders, such as myotilin, lamin A/C or caveolin 3 (Table 1).
Key words: Limb-girdle muscular dystrophies, LGMD, NGS
LGMD1A - LGMD1A may be caused by mutations in the myotilin (MYOT) gene at chr. 5q31.2. The cDNA is of 2.2 kb and contains 10 exons. Myotilin is a Z-diskassociated protein. LGMD1A may be considered as an occasional form of LGMD (4). The first clinical report was in 1994 (5). The gene was identified in 2000 (6), but myotilin mutations have been rather associated with myofibrillar myopathy. LGMD1A is characterized by late
Introduction The term limb-girdle muscular dystrophy refers to a long list of Mendelian disorders characterized by a progressive deterioration of proximal limb muscles. Very often, other muscles are affected, together with the heart
Address for correspondence: Vincenzo Nigro, via Luigi De Crecchio 7, 80138 Napoli, Italy; Telethon Institute of Genetics and Medicine (TIGEM), via Pietro Castellino 111, 80131 Napoli, Italy. - E-mail: [email protected]
Vincenzo Nigro and Marco Savarese
Table 1. Autosomal dominant limb girdle muscular dystrophy. Gene
Protein (protein function)
myotilin (structural; Z disc)
Allelic disorders (OMIM, #) Myopathy, myofibrillar, 3 (609200) Myopathy, spheroid body (182920) Cardiomyopathy, dilated, 1A(115200) Charcot-Marie-Tooth disease, type 2B1(605588) Emery-Dreifuss muscular dystrophy 2, AD(181350) Emery-Dreifuss muscular dystrophy 3, AR(181350)
lamin A/C (structural; fibrous nuclear lamina )
Heart-hand syndrome, Slovenian type(610140)
Hutchinson-Gilford progeria(176670) Lipodystrophy, familial partial, 2(151660) Malouf syndrome(212112) Mandibuloacral dysplasia(248370) Muscular dystrophy, congenital(613205) Restrictive dermopathy, lethal(275210) Cardiomyopathy, familial hypertrophic(192600)
caveolin 3 (scaffolding protein within caveolar membranes)
Creatine phosphokinase, elevated serum(123320) Childhood
Long QT syndrome 9(611818) Myopathy, distal, Tateyama type(614321) Rippling muscle disease(606072)
DnaJ/Hsp40 homolog, subfamily Variable B, member 6 (25-50y) (chaperone)
Muscular dystrophy, limbgirdle, type 2R(615325)
desmin (structural; intermediate filament)
Cardiomyopathy, dilated, 1I(604765) Adulthood
Myopathy, myofibrillar, 1(601419) Scapuloperoneal syndrome, neurogenic, Kaeser type(181400)
transportin 3 (nuclear importin)
Heterogeneous nuclear ribonucleoprotein D-like protein (ribonucleoprotein, RNA-processing pathways)
Genetic basis of limb-girdle muscular dystrophies: the 2014 update
onset proximal weakness with a subsequent distal weakness. Some patients show nasal and dysarthric speech. Serum CK is normal or mildly elevated. Muscle pathology shows rimmed vacuoles with or without inclusions. Electron microscopy shows prominent Z-line streaming. Cardiac and respiratory involvement occasionally occurs.
form. LGMD1D is caused by heterozygous missense mutations in the DNAJB6 gene at chr. 7q36.3 (10). The reference cDNA sequence is 2.5kb-long, contains 10 exons and encodes DnaJ homolog, subfamily B, member 6. DNAJ family members are characterized by a highly conserved amino acid stretch (2) called the ‘J-domain’. They exemplify a molecular chaperone functioning in a wide range of cellular events, such as protein folding and oligomeric protein complex assembly (11). Missense heterozygous mutations of DNAJB6 (p.Phe89Ile, p.Phe93Leu and p.Pro96Arg) are all located in the Gly/ Phe-rich domain of DNAJB6 leading to insufficient clearance of misfolded proteins. Functional testing in vivo have shown that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies have demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. DNAJB6 is located in the Z line and interacts with BAG3. Mutations in BAG3 are known to cause myofibrillar myopathy (12). A characteristic pathological finding of LGMD1D is the presence of autophagic vacuoles and protein aggregation. These protein aggregations contain DNAJB6 together with its known ligands MLF1 and HSAP1, and also desmin, αB-crystallin, myotilin, and filamin C, which are known to aggregate in myofibrillar myopathy. These results suggest that the phenotype of LGMD1D also overlaps with that of myofibrillar myopathy. LGMD1D patients show mildly elevated serum CK levels. The lower limbs are more affected, particularly the soleus, adductor magnus, semimembranosus and biceps femoris. In contrast, the rectus femoralis, gracilis and sartorius and the anterolateral lower leg muscles are mostly spared. DNAJB6 gene mutations may also be associated with distal-predominant myopathy. Symptoms in the upper limbs appear later. Some patients develop calf hypertrophy. Onset ranges from 25 to 50 years, with some patients maintaining ambulation throughout life. No cardiac or respiratory involvement has been reported so far. The pattern of differential involvement could be identified at different stages of the disease process.
LGMD1B - LGMD1B is also an occasional LGMD form caused by lamin A/C (LMNA) gene mutations at chr. 1q22 (7). The reference cDNA is of 3 kb and contains 12 exons. The LMNA gene gives rise to at least three splicing isoforms (lamin A, C, lamin AΔ10). The two main isoforms, lamin A and C, are constitutive components of the fibrous nuclear lamina and have different roles, ranging from mechanical nuclear membrane maintenance to gene regulation. The ‘laminopathies’ comprise different well-characterized phenotypes, some of which are confined to the skeletal muscles or skin, while others are multi-systemic, such as lipodystrophy, Charcot-Marie Tooth disease, progeroid syndromes, dilated cardiomyopathy and Emery-Dreifuss muscular dystrophy (EDMD). The LGMD1B is characterized by a symmetric proximal weakness starting from the legs, associated with atrioventricular conduction disturbances and dysrhythmias. CK is normal to moderately elevated. Most patients develop proximal leg weakness, followed by cardiac arrhythmias and dilated cardiomyopathy, with sudden death 20-30 years later. However, there is a continuity between LGMD1B and EDMD (8). Usually the more severe forms of EDMD with a childhood onset have missense mutations, whereas the milder LGMD1B is associated with heterozygous truncating mutations: this may arise through a loss of LMNA function secondary to haploinsufficiency, whereas dominant-negative or toxic gain-of-function mechanisms may underlie the EDMD phenotypes. LGMD1C - LGMD1C is caused by mutations in the caveolin 3 gene (CAV3) at chr. 3p25.3. The CAV3 gene encodes a 1.4kb mRNA composed of only two exons. Caveolin-3 is a muscle-specific membrane protein and the principal component of caveolae membrane in muscle cells in vivo: at present this is the only gene in which mutations cause caveolinopathies (9). LGMD1C is characterized by an onset usually in the first decade, a mild-to-moderate proximal muscle weakness, calf hypertrophy, positive Gower sign, and variable muscle cramps after exercise.
LGMD1E - For the limb girdle muscular dystrophy originally linked to chr. 6q23 (13) we will use the name LGMD1E, even if it should be considered, more correctly, as a form of autosomal dominant desminopathy or myofibrillar myopathy. This form is also known as dilated cardiomyopathy type 1F (CMD1F). One family previously categorized as having LGMD and dilated cardiomyopathy was reported, indeed, to have the splice site mutation IVS3+3A>G in the desmin (DES) gene at 2q35 (14).
LGMD1D - Autosomal dominant LGMD mapped to 7q36 has been classified as LGMD1E in OMIM, but as LGMD1D in the Human Gene Nomenclature Committee Database. In the literature there is another LGMD1D/E erroneously mapped to 6q, but we will use the acronym LGMD1D for the 7q-disease and LGMD1E for the 6q-
Vincenzo Nigro and Marco Savarese
Autosomal recessive LGMD
For desmin see also LGMD2R. As in the desminopathies, LGMD1E family members show dilated cardiomyopathy and conduction defects together with progressive proximal muscle weakness starting in the second or third decade. Some family members had a history of sudden death. Serum creatine kinase is mildly elevated (150-350U/l). Muscle pathology may show dystrophic changes, but later the presence of abundant perinuclear or subsarcolemmal granulofilamentous inclusions have been also observed. The study of these inclusions by laser capture microdissection followed by mass spectrometry analysis, led to the identification of the disease-causing mutations in desmin (14).
The autosomal recessive forms (LGMD2) are much more common, having a cumulative prevalence of 1:15,000 (2) with some differences among countries, depending on the carrier distribution and the degree of consanguinity. There are recessive genes in which the loss-of-function mutations on both alleles tipically result in a LGMD phenotype (ordinary LGMD genes): they correspond to the first 8 forms of LGMD2 (LGMD2A-2H) plus LGMD2L. On the contrary, other genes (occasional LGMD genes) show a phenotypic divergence with some mutations associated with LGMD and other ones determining a more complex disorder. Specific variations in occasional LGMD genes cause the other forms (LGMD2I-2U). The best examples come from dystroglycanopathies in which the LGMD presentation is associated with milder alleles of genes mutated in congenital forms with brain involvement (Table 2).
LGMD1F - LGMD1F was originally mapped to a 3.68-Mb interval on chromosome 7q32.1-7q32.2 in a very large Italo-Spanish family. We presented the identification of TNPO3 by whole exome sequencing of four affected family members and the complete refining of the region at the WMS 2012. Data were then published (15): a frame-shift mutation in the transportin 3 (TNPO3) gene is shared by all affected family members with 94% penetrance. The TNPO3 gene is composed of 23 exons and encodes a 923-amino acid protein, also expressed in skeletal muscle. The frame-shifted TNPO3 protein is larger than the wt, since it lacks the predicted stop codon and is found around the nucleus, but not inside. Patients with an onset in the early teens, show a more severe phenotype with a rapid disease course, while adult onset patients present a slower course. They have a prominent atrophy of lower limb muscles, involving especially the vastus lateralis and the ileopsoas muscle (16). Interestingly, some patients present with dysphagia, arachnodactyly and respiratory insufficiency. CK range is 1-3x. No cardiac involvement has been reported.
LGMD2A - LGMD2A is caused by Calpain 3 (CAPN3) gene mutations and represents the most frequent LGMD worldwide (20, 21). The CAPN3 gene spans 53kb of genomic sequence at chromosome 15q15.2 and the transcript is composed of 24 exons encoding a 94kDa muscle-specific protein. There is a number of heterozygotes (1:100), carrying many different CAPN3 pathogenic changes. Calpains are intracellular nonlysosomal cysteine proteases modulated by calcium ions. A typical calpain is a heterodimer composed of two distinct subunits, one large (> 80 kDa) and the other small (30 kDa). While only one gene encoding the small subunit has been demonstrated, there are many genes for the large one. CAPN3 is similar to ubiquitous Calpain 1 and 2 (m-calpain and micro-calpain), but contains specific insertion sequences (NS, IS1 and IS2). Calpains cleave target proteins to modify their properties, rather than “break down” the substrates. The phenotypic spectrum of calpainopathies is very broad, but they are true LGMD. For the clinical course, see also (1).
LGMD1G - LGMD1G has been mapped to chr. 4q21. Very recently, the defect in the RNA processing protein HNRPDL has been identified (17) in two different families by whole exome sequencing. The HNRPDL gene contains 8 exons and is ubiquiously expressed. The gene product is a heterogeneous ribonucleoprotein family member, which participates in mRNA biogenesis and metabolism. The reduced hnrpdl in zebrafish prodeces a myopathic phenotype. Patients show late-onset LGMD associated with progressive fingers and toes flexion limitation (18).
LGMD2B - It is caused by missense or null alleles of the dysferlin (DYSF) gene (22). The DYSF gene spans 233kb of genomic sequence at chr. 2p13.2 and the major transcript is composed of 6,911 nt containing 57 exons in the HGVS recommended cDNA Reference Sequence. Dysferlin is an ubiquitous 230-KDa transmembrane protein involved in calcium-mediated sarcolemma resealing. LGMD2B is the second most frequent LGMD2 form (1525%) in numerous countries, but not everywhere (23). Muscle inflammation is recognized in dysferlinopathy and dysferlin is expressed in the immune cells.
LGMD1H - By studying a large pedigree from Southern Italy, a novel LGMD locus has been mapped on chromosome 3p23-p25.1 (19). Most of patients present with a slowly progressive proximal muscle weakness, in both upper and lower limbs, with onset during the fourthfifth decade of life.
Genetic basis of limb-girdle muscular dystrophies: the 2014 update
Table 2. Autosomal recessive limb girdle muscular dystrophy. Gene
LGMD Typical onset Progression Cardiomiopathy phenotype ordinary
Allelic disorders (OMIM, #)
3–20X Miyoshi muscular dystrophy 1 (254130)
Cardiomyopathy, dilated, 1L (606685)
Cardiomyopathy, dilated, 1N (607487)
Tripartite motif containing 32
Bardet-Biedl syndrome 11 (209900)
Fukutin related protein
Myopathy, distal, with anterior tibial onset (606768)
Cardiomyopathy, dilated, 1G (604145) Cardiomyopathy, familial hypertrophic, 9 (613765) LGMD2J
312 or more
Myopathy, early-onset, with fatal cardiomyopathy (611705) Myopathy, proximal, with early respiratory muscle involvement (603689) Tibial muscular dystrophy, tardive (600334) Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 1 (236670)
Protein-O-mannosyl transferase 1
Muscular dystrophy10-40X dystroglycanopathy (congenital with mental retardation), type B, 1 (613155) Muscular dystrophydystroglycanopathy (limbgirdle), type C, 1 (609308)
Variable (young to late adulthood)
Gnathodiaphyseal dysplasia (166260) Miyoshi muscular dystrophy 3 (613319) Cardiomyopathy, dilated, 1X (611615)
occasional Early childhood
Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 4 10-70X (253800) Muscular dystrophydystroglycanopathy (congenital without mental retardation), type B, 4 (613152)
Vincenzo Nigro and Marco Savarese
Table 2. (follows). Gene Disease
Protein-O-mannosyl transferase 2
Protein O-linked mannose beta1,2-Nacetylglucosaminyl transferase
LGMD Typical onset Progression Cardiomiopathy phenotype
occasional Early childhood
Allelic disorders (OMIM, #) Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 2 (613150) Muscular dystrophydystroglycanopathy (congenital with mental retardation), type B, 2 (613156) Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 3 (253280)
occasional Late childhood
Muscular dystrophydystroglycanopathy (congenital with mental retardation), type B, 3 (613151) Muscular dystrophydystroglycanopathy (limbgirdle), type C, 3 (613157)
20X Epidermolysis bullosa simplex with pyloric atresia (612138)
Epidermolysis bullosa 10-50X simplex, Ogna type (131950) Muscular dystrophy with epidermolysis bullosa simplex (226670) Muscular dystrophy, limbgirdle, type 2R(615325)
Desmin (structural; intermediate filament)
A-V conduction block
Cardiomyopathy, dilated, 1I(604765) 1X
Myopathy, myofibrillar, 1(601419) Scapuloperoneal syndrome, neurogenic, Kaeser type(181400)
Transport protein particle complex 11
GDP-mannose pyrophosphorylase B
Isoprenoid synthase domain containing
Lim and senescent cell antigen-like domains 2
Early childhoodYoung adulthood
9-16X Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 14 (615350)
Muscular dystrophydystroglycanopathy (congenital with mental retardation), type B, 14 (615351)
Early / Late
Muscular dystrophydystroglycanopathy (congenital with brain and eye anomalies), type A, 7 (614643)
Variable (Rapid to slow)
Glycogen storage disease II (232300)
Genetic basis of limb-girdle muscular dystrophies: the 2014 update
named adhalin and contains a “dystroglycan-type” cadherin-like domain that is present in metazoan dystroglycans (35). LGMD2E - The beta-sarcoglycan gene spans 15kb of genomic sequence at chromosome 4q11 and the major transcript is composed of 6 exons. The protein contains of 318 amino acids and weighs 43kDa. LGMD2F - Delta-sarcoglycan is by far the largest LGMD gene, spanning 433kb of genomic sequence at chromosome 5q33.3 and the major transcript is composed of 9 exons. Intron 2 alone spans 164kb, one the largest of the human genome. Delta and gamma sarcoglycan are homologous and of identical size (35kDa). LGMD2G - Mutations in titin cap (Tcap)/Telethonin cause LGMD2G, one of the rarest forms of LGMD (36). Tcap provides links to the N-terminus of titin and other Zdisc proteins. Patients show adolescence-onset weakness initially affecting the proximal pelvic muscles and then the distal legs with calf hypertrophy. A homozygous nonsense mutation in the Tcap gene has been described in patient a congenital muscular dystrophy. The Tcap gene has also been associated with cardiomyopathy (37), while common variants may play a role in genetic susceptibility to dilated cardiomyopathy. Immunofluorescence and Western blot assays may show a Telethonin deficiency. Full sequencing testing may be cost-effective in all cases, because the gene is composed only of two small exons. The telethonin gene (TCAP) spans 1.2kb of genomic sequence at chromosome 17q12 and the transcript is composed of 2 exons. The protein product is a 19kDa protein found in striated and cardiac muscle. It binds to the titin Z1-Z2 domains and is a substrate of titin kinase, interactions thought to be critical for sarcomere assembly. Only two different mutations have been described in the TCAP gene in Brazilian patients (36). A mutation (R87Q) was found in a patient with dilated cardiomyopathy (37). Moreover, a human muscle LIM protein (MLP) mutation (W4R) associated with dilated cardiomyopathy (DCM) results in a marked defect in Telethonin interaction/localization (38). LGMD2H - The Tripartite-motif-containing gene 32 (TRIM32) gene spans 14kb of genomic sequence at chromosome 9q33.1 and the transcript is composed of 2 exons, with the first noncoding and the second encoding a 673 aa protein of 72kDa. TRIM32 is a ubiquitous E3 ubiquitin ligase that belongs to a protein family comprising at least 70 human members sharing the tripartite motif (TRIM). The TRIM motif includes three zinc-binding domains, a RING, a B-box type 1 and a B-box type 2, and a coiled-coil region. The protein localizes to cytoplasmic bodies. Although the function of TRIM32 is unknown, analysis of the domain structure of this protein suggests that it may be an E3-ubiquitin ligase (39).
The “dysferlinopathies” include limb-girdle muscular dystrophy type 2B (LGMD2B) and the allelic forms Miyoshi myopathy (MM), which is an adult-onset distal form, and distal myopathy with anterior tibialis onset (DMAT), but varied phenotypes are observed. LGMD2B affects earlier the proximal muscles of the arms whereas MM affects the posterior muscles of the leg. DYSF gene mutations are associated with heterogeneous clinical pictures ranging from severe functional disability to mild late-onset forms (24). About 25% of cases are clinically misdiagnosed as having polymyositis (25). This classification into separate phenotypes does not reveal true disease differences (26) and the allelic forms are not due to different mutations. Additional factors (e.g., additional mutations in neuromuscular disease genes or sport activities that include maximal eccentric contractions) may worsen the disease expression of causative mutations in dysferlinopathies (27). WB analysis is very useful and specific (28) when G), resulting in the addition of 16 amino acids from residue 428. Since then, other mutations have been identified. The patients have onset in their teens or twenties of progressive proximal muscle weakness and non-specific atrophy affecting both the upper and lower limbs. The serum Ck is normal. LGMD2R patients usually show A-V conduction blocks but no cardiomyopathy. LGMD2S - This is caused by mutation in the transport protein particle complex 11 (TRAPPC11) gene that spans 54,328 bp at chr. 4q35, the mRNA is 4.5kb and contains 30 exons. Recently, mutations in TRAPPC11 have been identified in a consanguineous Syrian family with an uncharacterized form of LGMD and in five Hutterite individuals presenting with myopathy, ID, hyperkinetic movements and ataxia (78). TRAPPC11 is a transport protein particle component involved in anterograde membrane transport from the endoplasmic reticulum (ER) to the ER-to-Golgi intermediate compartment (ERGIC) in mammals (79). Mutations identified so far (c.2938G>A/ p.Gly980Arg and c.1287+5G>A) cause modifications in TRAPP complex composition, in Golgi morphology and in cell trafficking. The LGMD2S pathogenic mechanism is similar to that causing Danon disease, an X-linked myopathy due to LAMP2 mutations and affecting the secretory pathway (80). The LGMD2S phenotype ranges from a slowly progressive LGMD with childhood onset and high CK to a syndrome characterized by myopathy but also neurological involvement (ID and ataxia). LGMD2T - LGMD2T is caused by milder mutations in the GDP-mannose pyrophosphorylase B (GMPPB) gene (81). The GMPPB gene is a small gene of 2,453bp at chr. 3p21. The mRNA is 1.7kb and contains 8 exons. Mutations in the GMPPB gene have been associated with congenital muscular dystrophies with hypoglycosylation of α-dystroglycan and also with LGMD only in three un-
cardiomyopathy or respiratory involvement. LGMD2L is allelic with the AD gnathodiaphyseal dysphasia (63) and with AR distal myopathy (MMD3) (64). LGMD2M - This is associated with mutations in the fukutin gene (FKTN) at chr. 9q31.2 (65). The FKTN gene spans 82,989 bp and contains 10 coding exons, the main transcript is 7.4kb encoding a protein of 413 amino acids. Also in this case LGMD2M is a milder form caused by at least one hypomorphic missense mutation in a gene that, with both non-functional alleles, is associated with more severe phenotypes (66): WWS, MEB or congenital muscular dystrophies (67). In LGMD2M the CNS is not affected and the intelligence is normal. Patients are hypotonic, may be ambulant and the onset is in early childhood. They show symmetric and diffuse muscle involvement that deteriorates with acute febrile illness. Improvement is seen with steroids. CK levels are 10-50x. There is also evidence for spinal rigidity, contractures and cardiomyopathy and respiratory involvement. LGMD2N - Mutations in the POMT2 gene, containing 21 exons, at chr. 14q24 cause LGMD2N (68). POMT2 is a second O-mannosyltransferase overlapping with POMT1 expression. POMT2 mutations usually have a dramatic effect: they cause Walker-Warburg syndrome or muscle-eye-brain-like (69), but rarely are associated with LGMD (70). This may occur when the α-dystroglycan glycosylation is only slightly reduced. In these cases the mutations are usually missense and the phenotype is characterized by LGMD without brain involvement, very high serum CK. LGMD2O - It is associated with milder mutations in the POMGnT1 gene at chr. 1p32 (71). Usually mutations in the POMGnT1 gene are associated with more severe phenotypes than LGMD, such as Walker-Warburg syndrome or MEB. A homozygous hypomorphic allele of the POMGnT1 gene was found as a 9-bp promotor duplication (72). LGMD2P - LGMD2P is caused by specific changes of the dystroglycan (DAG1) gene itself. Recently, Campbell has reported a missense mutation in the dystroglycan gene in an LGMD patient with cognitive impairment (73). This substitution interferes with LARGE-dependent maturation of phosphorylated O-mannosyl glycans on α-dystroglycan affecting its binding to laminin. As a rule the dystroglycanopathies are due to mutations in genes involved in the glycosylation pathway of dystroglycan, but the dystroglycan gene is normal. LGMD2Q - This form of LGMD is mutation-specific since other mutations in the Plectin (PLEC1) gene at chrom. 8q24.3 cause epidermolysis bullosa simplex (74). LGMD2Q has been identified as a homozygous 9-bp deletion in consanguineous Turkish families (75). The deletion affects an AUG that is only present in a muscle-
Vincenzo Nigro and Marco Savarese
related patients so far reported. The patients from Indian and Egyptian descent presented with microcephaly and intellectual delay. All 3 patients had increased serum creatine kinase and dystrophic findings on muscle biopsy. Muscle biopsy showed hypoglycosylation of DAG1. The English LGMD patient was a 6-year-old boy with exercise intolerance and CK = 3,000 UI. Two missense mutations were identified: pAsp27His and p.Val330Ile. LGMD2U - This is the form caused by some particular alleles of the isoprenoid synthase domain containing (ISPD) gene. The ISPD gene spans 333kb at chromosome 7p21 and contains 10 exons. ISPD mutations disrupt dystroglycan mannosylation and cause of WalkerWarburg syndrome (82, 83). Mutations in ISPD as well as TMEM5 genes have been associated with severe cobblestone lissencephaly (84). Null alleles of ISPD produce Walker Warburg or cobblestone lissencephaly with brain vascular anomalies, but at least one milder mutation in one allele has been found in LGMD (68 69). We named this forms as LGMD2U. The association between mutations in the ISPD gene and LGMD was, however, older than that of forms 2P-2T, but to avoid discordant definitions among the LGMD2U should be considered as that caused by some alleles of ISPD. LGMD2U is progressive, with most cases with LGMD losing ambulation in their early teenage years, thus following a DMD-like path. In several patients, there is muscle pseudohypertrophy, including the tongue. Respiratory and cardiac functions also decline, resembling other dystroglycanopathies. LGMD2V - This is a proposal to name as LGMD2V an occasional LGMD form that derives from mild mutations of the acid alpha-glucosidase (GAA) gene (85). The GAA gene maps at chr 17q25.3 and comprises 20 exons with a protein product of 953 aa. Defects in GAA are the cause of glycogen storage disease type 2 (GSD2, MIM: 232300). GSD2 is a metabolic disorder with a broad clinical spectrum. The severe infantile form, or Pompe disease, presents at birth with massive accumulation of glycogen in muscle, heart and liver. Late-onset Pompe disease may present from the second to as late as the seventh decade of life with progressive proximal muscle weakness primarily affecting the lower limbs, as in a limb-girdle muscular dystrophy. Final outcome depends on respiratory muscle failure. LGMD2W - This caused by mutations in the LIM and senescent cell antigen-like-containing domain protein 2 (LIMS2/ PINCH2) gene at chromosome 2q14. The gene comprises 7 coding exons. It encodes a 341-aa member of a small family of focal adhesion proteins. The encoded protein has five LIM domains, each domain forming two zinc fingers, which permit interactions which regulate cell shape and migration. Patients show a childhood onset LGMD with macroglossia and calf enlargement. They al-
so developed decreased ejection fraction with global left ventricular dysfunction in their 3rd decade, severe quadriparesis and relative sparing of the face, and characteristically a broad based triangular tongue. This form has been presented in a poster session at the ASHG 2013. The classification of LGMD is becoming too complex. We tried to reorganize the different genes so far described following the traditional nomenclature. However for the autosomal recessive forms there are few letters available. The next forms will be LGMD2X, LGMD2Y and LGMD2Z. We propose, after the LGMD2Z form, the acronyms LGMD2AA, LGMD2AB, LGMD2AC, etc. to avoid renaming consolidated definitions thereby generating even higher confusion.
Acknowledgements This study was mainly supported by grants from Telethon, Italy (TGM11Z06 to V.N.) and TelethonUILDM (Unione Italiana Lotta alla Distrofia Muscolare) (GUP 10006 and GUP11006 to V.N.). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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